J. Phys. Chem. B 2006, 110, 19315-19318
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High-Temperature Seedless Synthesis of Gold Nanorods Peter Zijlstra,* Craig Bullen, James W. M. Chon,* and Min Gu Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne UniVersity of Technology, P.O. Box 218, Hawthorn, 3122 VIC, Australia ReceiVed: June 8, 2006; In Final Form: August 1, 2006
We demonstrate seedless synthesis of gold nanorods at high temperatures up to 97 °C. Using the correct silver nitrate concentration is crucial for formation of rod-shaped particles at all temperatures. We observed a decrease of nanorod length with increasing temperature, while the width stays constant throughout the temperature range. From kinetics studies, we show 3 orders of magnitude increase in nanorod growth rate when the temperature is raised from room temperature to 97 °C. From the temperature dependence of the growth rate, we obtain a average activation energy for growth on all facets of 90 ( 10 kJ mol-1. Hightemperature synthesis of gold nanorods presents a more attractive method for scalable flow-based production of gold nanorods.
Introduction Gold nanoparticles have received much attention recently because of their strong absorption at frequencies in the visible spectrum. This absorption band is a result of the collective oscillation of free electrons and is called a surface plasmon resonance.1 In particular, anisotropic gold nanoparticles such as nanorods have been the subject of recent research.2 The longitudinal surface plasmon (LSP) absorption band of gold nanorods can be tuned throughout the visible spectrum by tuning the aspect ratio. Recently, it was reported that the LSP drastically enhances the photoluminescence of gold nanorods as compared to bulk gold,3 making them an excellent candidate for biological labeling and imaging.4,5 Alternatively, melting of gold nanorods under pulsed laser irradiation6,7 has demonstrated the viability of optical data storage in gold nanorods.8 Many groups have reported methods to synthesize gold nanorods, including templated synthesis,9 photochemical10 and electrochemical methods,11 and wet-chemical synthesis.12,13 All of these reported methods involve synthesis at temperatures close to room temperature. Jana et al.14 recently concluded for their system that cetyl-trimethylammonium bromide (CTAB) micelles break down at high temperature and indicated that nanorods only form at temperatures below 50 °C. Similarly, Pe´rez-Juste et al.15 reported that reaction temperatures close to room temperature are more favorable for higher nanorod yields. In this article, we present a kinetics study of seedless nanorod synthesis that is viable at high temperatures. The reaction temperature was varied between 25 and 97 °C. We found that using the correct silver nitrate concentration is crucial for the formation of rod-shaped particles at high temperatures. Transmission electron microscopy (TEM) images of the final products demonstrate a decrease in rod length at high temperatures while the width stays constant. We observed an increase in growth rate of 3 orders of magnitude when the temperature was raised from room temperature to 97 °C, presenting a more attractive method for large-scale production of gold nanorods. Additionally, from the temperature dependence of the growth rate, we * Corresponding authors. E-mail:
[email protected] (P.Z.); JChon@ groupwise.swin.edu.au (J.W.M.C.).
obtain the average activation energy for the growth on all nanorod facets. To perform a systematic study of the effect of temperature on rod growth, the initial conditions for each reaction should be identical. This is hard to accomplish when using a seeded growth method because the properties of seed solutions are not easily reproducible and are also temporally labile. To avoid these effects, a seedless growth method comparable to the method previously reported by Jana et al.14 was adopted for this study. A seedless growth method involves in situ formation of seeds, in contrast to seeded growth where seeds are formed ex situ. Experimental Section Particle Synthesis. Gold nanorods were prepared using a seedless growth method in aqueous solution, comparable to the method previously reported by Jana et al.14 A 5 mL aq solution containing CTAB (0.1 M) and HAuCl4 (0.5 mM) was prepared. Various concentrations of AgNO3 were added to this solution, depending on the experiment. Au(III) was reduced to Au(I) by adding 30 µL of an ascorbic acid solution (0.1 M) under vigorous stirring. Nucleation and growth were initiated 5 s after adding the ascorbic acid by quickly injecting 2 µL of a NaBH4 solution (1.6 mM) under vigorous stirring. Temperature Control. Temperature control during the nucleation and growth of the particles was achieved by immersing the sample vials in a heat bath. Depending on the experiment, a water bath (Memmert WB-10) or an oil bath was used. The vials were immersed in the heat bath throughout the experiment (including injection of ascorbic acid and NaBH4) to ensure a constant temperature during the growth. Temperatures listed in the next section are temperatures of the growth solution just before the injection of ascorbic acid. Vials were kept in the heat bath until the end of the reaction and at room temperature thereafter. Instrumentation. Absorption spectra of the final solutions were measured using a Shimadzu UV-1601 spectrophotometer with a path length of 1 cm. The absorption spectra we obtained periodically during the growth of the nanorods were measured using an Ocean Optics HR2000CG spectrophotometer with a dip probe (Ocean Optics T300-RT-UV-VIS). The path
10.1021/jp0635866 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006
19316 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Figure 1. Photographs of reaction products obtained as a function of silver nitrate concentration and temperature. The optimum silver nitrate concentration is highlighted. A blue color indicates formation of short nanorods of aspect ratio ∼3, a brownish color is caused by nanorods with aspect ratio ∼4-5 ,whereas a red color indicates the presence of spheres in the solution.
length of the dip probe was 2 mm, however, all absorption spectra shown here were renormalized to a path length of 1 cm. Preparation of TEM Samples. After the reaction finished, the reaction product was centrifuged at 18 500g for 10 min to separate the nanorods from excess surfactant. The pellet was redispersed in distilled water. TEM samples were prepared by placing 2 µL of the redispersed solution on a copper grid. TEM images were acquired with a Philips CM120 BioTWIN transmission electron microscope. Results We start by describing the effect of the silver concentration and reaction temperature on the final products of the synthesis. Next, we discuss the effect of the reaction temperature on the kinetics of gold nanorod synthesis. The Effect of [AgNO3] and Temperature on Nanorod Synthesis. The inclusion of AgNO3 in the growth solution was found to be essential for nanorod synthesis at elevated temperatures. Silver nitrate is believed to slow the growth of the nanorod, resulting in better control over the final aspect ratio. Liu et al. recently reported that silver acts as a surface-structurespecific surfactant.16 They concluded that silver deposits selectively on the more open {110} facet of the gold rod, slowing down the growth of this facet. In Figure 1, we show photographs of the final reaction products we obtained by varying temperature and silver nitrate concentration. A solution with a blue color indicates formation of short nanorods with an LSP around 700 nm (aspect ratio ∼3). A brownish color is from nanorods absorbing in the nearinfrared (aspect ratio 4-5). A red color indicates the presence of spheres in the solution. As can be seen from Figure 1, choosing an optimum silver nitrate concentration becomes crucial at higher temperatures. High silver nitrate concentration resulted in a low rod yield and mainly spheres at temperatures above 50 °C. A low silver nitrate concentration resulted in short rods throughout the temperature range. The optimum silver
Zijlstra et al.
Figure 2. Absorption spectra of gold nanorod solutions ([AgNO3] ) 0.12 mM) synthesized at different temperatures; the spectra are offset for clarity. The normalization factors are: 1.27 for 25 °C, 1.93 for 30 °C, 1.70 for 35 °C, 1.92 for 40 °C, 1.20 for 50 °C, 1.57 for 60 °C, 1.77 for 80 °C, and 1.95 for 97 °C, respectively.
nitrate concentration (0.12 mM, highlighted in Figure 1) resulted in formation of nanorods throughout the temperature range. We used the optimum silver nitrate concentration in the remainder of the experiments described here. In Figure 2 we show the absorption spectra of the solutions synthesized at different temperatures ([AgNO3] ) 0.12 mM). All samples had a final peak absorption >1. The absorption spectra were normalized to the peak value, and the normalization factors are indicated in the caption. The spectra are offset for clarity. It is apparent from the absorption spectra that the amount of spherical byproducts with absorption
